1. Field of the Invention
The present invention relates to a power control method for a mobile communication system, and more particularly to an apparatus and a method for controlling power by means of an open-loop scheme in a mobile communication system using a Time Division Duplex (TDD) scheme.
2. Description of the Related Art
From the end of the 1970's, at which time a cellular type wireless mobile communication system was developed in United States, a voice communication service has been provided to users through an Advanced Mobile Phone Service (AMPS) system, which has been referred to as a 1st generation (1G) analog type mobile communication system. Then, in the middle of the 1990's, a Code Division Multiple Access (CDMA) system has been commercialized, and is commonly referred to as a 2nd generation (2G) mobile communication system, so that a voice service and a low speed data service have been provided.
In addition, an International Mobile Telecommunication-2000 (IMT-2000), which is commonly referred to as a 3rd generation (3G) mobile communication system, was proposed at the end of the 1990's for the purpose of providing an improved wireless multimedia service, a worldwide roaming service and a high-speed data service. Recently, the IMT-2000 services have been partially provided to users. In particular, the 3G mobile communication system has been developed to transmit higher speed data to handle the rapid increase of the amount of data provided from a mobile communication system. The 3G mobile communication system has been developed into a packet service communication system. The packet service communication system is a system for transmitting burst packet data to a plurality of mobile stations and is well suited for the transmission of mass storage data. The packet service communication system is being developed for high speed packet services.
Currently, the 3G mobile communication system is being developed into a 4th generation (4G) mobile communication system. Apart from the previous mobile communication systems that provide only wireless communication services, the 4G mobile communication system is being standardized for the purpose of providing integrated wired/wireless communication services through an effective combination of a wireless communication network and a wire communication network. Accordingly, it is necessary to develop technology capable of transmitting mass storage data at levels compatible to the capacity of a wire communication network in a wireless communication network.
Hereinafter, the network construction of the mobile communication system as described above will be described with reference to FIG. 1.
FIG. 1 is a block diagram showing the network construction of a mobile communication system according to the prior art.
Referring to FIG. 1, a general 3G mobile communication system includes a Core Network (CN) 101, a base station controller 103, a plurality of base stations 105, and a plurality of subscriber stations (SSs) 107.
The CN 101 is connected to the base station controllers 103 by wire and each of the base station controllers 103 is connected to the base stations 105 by wire. Further, each of the base stations 105 transmits/receives through a wireless link data to/from one or more subscriber stations 107, which belong to an area controlled by the base station 105.
The CN 101 plays an independent role in a wireless access technology and controls position management, identification, call connection, etc., of the subscriber stations 107. The base station controller 103 controls radio resources to be assigned to the base station 105 connected to the base station controller 103. That is, the base station 105 transmits common broadcast signals to plural subscriber stations through downlink channels, which exist in a cell controlled by the base station 105, and transmits a specific signaling or user traffic to each of the subscriber stations. Further, the base station 105 receives and processes signaling transmitted from the subscriber station through an uplink channel. When a call setup has been accomplished, the base station 105 receives signals transmitted from each subscriber station.
Herein, an uplink channel and a downlink channel exchanged on a wireless link between the base station 105 and the subscriber station 107 may be transmitted/received through a TDD scheme or a Frequency Division Duplex (FDD) scheme.
The TDD scheme is a duplex scheme. That is, the duplex is a scheme for differentiating an uplink from a downlink between a subscriber station and a base station. The duplex scheme may be classified into an FDD scheme and a TDD scheme as described above. In the FDD scheme, an uplink is differentiated from a downlink by different frequencies and transmission antennas/reception antennas must independently exist in a subscriber station and a base station.
Different from the FDD scheme, in the TDD scheme, one antenna performs a transmission function and a reception function. In the TDD scheme, an uplink and a downlink exist as signals of the same frequency band. In order to differentiate the uplink from the downlink, which occupy the same frequency band in the TDD scheme as described above, an uplink signal and a downlink signal must be time-divided. That is, a time slot that includes the uplink signal and a time slot that includes the downlink signal are defined in advance, so that the uplink signal and the downlink signal can be communicated only during each time slot. In addition, the TDD scheme has increased circuit complexity as compared with the FDD scheme. However, the TDD scheme has high efficiency in use of frequencies. The construction of frames in a mobile communication system using the TDD scheme as described above will now be described.
FIG. 2 is a block diagram showing the construction of an uplink/downlink frame in the mobile communication system using the TDD scheme according to the prior art.
Referring to FIG. 2, in the TDD scheme, downlink frames 201 and 205 and an uplink frame 203 are time-divided in the same frequency band and then alternately and repeatedly transmitted. That is, the downlink frame 201 is transmitted and then the uplink frame 203 is transmitted after a predetermined transmission gap 213 passes. Similarly, the uplink frame 203 is transmitted and then the downlink frame 205 is transmitted after a predetermined transmission gap 219 passes.
The transmission gaps 213 and 219 assigned between the downlink and the uplink are gaps in which there are no signals and gaps established to prevent interference between signals which may occur due to sharing of the same frequency band by the uplink and the downlink.
A broadcasting channel 207 exists in the first portion of the downlink frame 201. The broadcasting channel 207 is a channel for transmitting system information used in controlling radio resources of a subscriber station. Downlink-bursts (DL-bursts) 209 and 211 for different subscriber stations sequentially exist after the broadcasting channel 207. Each subscriber station must receive downlink data during a time slot assigned for a channel of the subscriber station itself (i.e., during a corresponding downlink burst interval). Similarly, the uplink frame 203 includes a plurality of uplink-bursts (UL-bursts) 215 and 217 and each subscriber station must transmit uplink data during a time slot, that is, a corresponding uplink burst interval, which is assigned as the channel of the subscriber station.
In order to obtain the increase of communication capacity, high quality communication, etc., a general mobile communication system uses a downlink (direction from a base station to a subscriber station) power control and an uplink (direction from a subscriber station to a base station) power control. When the originated signal of a subscriber station is received by a base station with a signal-to-interference ratio (SIR) at a minimum required communication quality level, based on the transmit power control for all subscriber stations, a system capacity can be maximized. If the very strong signal of a subscriber station is received in a base station, the performance of the subscriber station is improved. However, this may cause an increase in the interference for another subscriber stations using the same channel. Therefore, the call quality of another subscriber station may be lowered below a predetermined level without the reduction of a maximum capacity.
A general CDMA communication system uses a forward (or downlink) open-loop power control method, a backward (or uplink) open-loop power control method and a backward closed-loop power control method as a method for the aforementioned power control. The forward power control is performed by a base station. That is, when a subscriber station is in an idle state or is relatively adjacent to a base station, has a small influence due to a multi-path fading and wave shadow, or experiences weak interference by another base station, the transmit power for a corresponding subscriber station is decreased. Additional power is provided to a subscriber station which is located in a bad reception area or in a position remote from a base station and thus has a high error rate.
The backward open-loop power control is performed by a subscriber station. The subscriber station measures power received from a base station, reflects downlink path loss and changes of a channel due to topography from the base station to the subscriber station in the magnitude of transmit power, and increases/decreases the magnitude of the transmit power, thereby compensating for the transmit power. In this way, transmission signals with the same intensities from all subscriber stations in a cell are received in a base station.
The backward closed-loop power control is a method by which subscriber stations control power at the command of a base station. The base station receives signals of relating subscriber stations, compares the signals with a predetermined target value, issues a power increase or decrease order to the subscriber stations with a predetermined time cycle, for example, every 1.25 ms. In this way, gain difference and different wave loss on a downlink and an uplink are compensated for.
In order to reduce the power consumption of a subscriber station and perform efficient communication, the aforementioned 3G mobile communication system and 4G mobile communication system use a power control method in which the systems control and transmit the transmit power of a base station or a subscriber station with a predetermined time cycle. Further, as described above, in the power control, a base station or a subscriber station adjusts the power PTx of a transmission signal so that the Signal-to-Noise Ratio (SNR) of a signal PRx arriving at a reception side maintains a predetermined target value.
Hereinafter, the open-loop power control method of the power control methods will be described in detail. In general, the TDD system uses the open-loop power control.
In such a case, a power determination method using an open-loop scheme is accomplished by compensating for power loss due to path loss. The path loss corresponds to a difference between the transmit power of a base station and the receive power of a subscriber station. That is, the subscriber station measures the power of received signals and the base station informs the subscriber station of the power of transmitted signals through a predetermined message. The subscriber station regards the difference between the two values as the path loss.
Hereinafter, a power control method in an asynchronous 3G mobile communication system, that is, a WCDMA system, will be described as one example of the power control method.
FIG. 3 is a flow diagram illustrating an open-loop power control method performed in a WCDMA-TDD mobile communication system according to the prior art.
First, a subscriber station receives a Primary Common Control Physical Channel (P-CCPCH) signal 301 or a downlink pilot signal at each frame from a base station, and measures the receive power PPCCPCH,rx of the P-CCPCH signal or the pilot signal (303). The physical layer of the subscriber station transmits to a Radio Resource Control (RRC) layer which is an upper layer (305) a System Information Block (SIB) including information on a system received from the base station.
A Radio Bearer (RB) setup is accomplished (307) between the subscriber station and the base station before a call setup is accomplished. The subscriber station reads a target SIR(SIRtarget), interference power (IBTS) measured in the base station, the transmission output value (PPCCPCH,tx) of a P-CCPCH and a power compensation value (DPCHconst) from an RRC RB Setup message received from the base station, and then initializes the physical layer through the values (309).
When the call setup is completed by the method as described above, the subscriber station transmits a first uplink frame, which is an initially transmitted uplink frame, at a power level calculated by a predetermined scheme (311). Herein, the transmit power of the first uplink frame is determined by the open-loop power control method and a method of determining the value of the transmit power is expressed in Equation 1.PDPCH=αLPCCPCH+(1−α)L0+IBTS+SIRtarget+DPCHconst  (1)
In Equation 1, PDPCH denotes the transmit power of the subscriber station and LPCCPCH denotes path loss experienced by the P-CCPCH signal. The path loss LPCCPCH may be calculated as a difference between a transmit power value in the base station and a receive power value in the subscriber station for the P-CCPCH. Further, the path loss LPCCPCH may be expressed by Equation 2.LPCCPCH=PPCCPCH,tx−PPCCPCH  (2)
In Equation 2, the PPCCPCH,tx, which is the transmit power value in the base station for the P-CCPCH, is transmitted from the base station to the subscriber station through a predetermined message. The PPCCPCH,rx, which is the receive power value in the subscriber station for the P-CCPCH, is obtained by measuring the receive power of the P-CCPCH signal received in the subscriber station.
Further, L0 in Equation 1 is a long time average value obtained by calculating an average during a predetermined time for the path loss LPCCPCH. Referring to Equation 1, the total path loss L is defined as a weighted average of the LPCCPCH and the L0. That is, the path loss is the first and the second term in Equation 1 and may be expressed by Equation 3.path loss=αLPCCPCH+(1−α)L0  (3)
In Equation 3, the α value is a value set for assigning a weighted value. If a time interval between an uplink and a downlink is small, the channel variation is small. Accordingly, it is preferred to set a large α value in order to increase the portion of the LPCCPCH. In contrast, if the time interval between the uplink and the downlink is large, the channel variation is large. Accordingly, it is preferred to set a small α value in order to increase the portion of the L0.
In Equation 1, IBTS is an interference power value measured by the base station. In the case of a general WCDMA system, since uplink signals of all subscriber stations experience the same interference according to each time slot, the IBTS is commonly applied to all subscriber stations according to each time slot. Further, the IBTS may be periodically transmitted through a broadcast channel.
The SIRtarget is a target value of an SIR which each subscriber station must obtain and is transmitted through a predetermined message before a dedicated physical channel is generated as described above. When it is necessary to renew the SIRtarget even after the dedicated physical channel has been generated, the SIRtarget is transmitted to a subscriber station through a predetermined message.
The DPCHconst is a power compensation value for fine power control in an open-loop power control. The conventional mobile communication system using an open-loop scheme uses the DPCHconst value as a constant value.
In relation to the 4G mobile communication system, an Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) scheme has been recently proposed. The power control as described above is a problem even in a system using the OFDM/OFDMA scheme.
The OFDM/OFDMA scheme is a scheme used in an IEEE 802.16 based system and a scheme of converting serial modulation symbols to parallel data and transmitting the parallel data. Further, the OFDM/OFDMA scheme is a multiple access scheme, uses a Time Division Multiple Access (TDMA) scheme, and uses a TDD scheme as a duplex scheme. In the case of the OFDM scheme, 256 modulation symbols are generally subjected to a Fast Fourier Transform (FFT), so that one OFDM symbol is formed. In the case of the OFDMA scheme, one OFDM symbol is formed using a much greater number of modulation symbols. Further, in the OFDMA scheme proposed in the IEEE 802.16, a subchannel is formed from subcarriers constituting one OFDM symbol and plural OFDM symbols constitute one frame. Hereinafter, the OFDMA system as described above will be described.
FIG. 4 is a diagram showing the construction of an uplink/downlink frame in the conventional 802.16 OFDMA system.
Referring to FIG. 4, each frame includes a plurality of bursts marked by quadrangles in a time-frequency plane. Each of the bursts are multiple-accessed between subscriber stations and a base station through a TDMA scheme. Further, the uplink frame and the downlink frame are duplexed through a TDD scheme and Transmission Gaps (TGs) referred to as a Transmit/Receive Transition Gap (TTG) and a Receive/Transmit Transition Gap (RTG) are provided between the uplink frame and the downlink frame.
Each subscriber station performs an initial ranging and a periodic ranging in order to correct time and frequency errors of each burst in the uplink frame and adjust power. When the subscriber station attempts the ranging process, the base station measures the signal power of the subscriber station and informs the subscriber station of a compensation value for signal power loss due to path loss and rapid change of the signal power through a message of a Medium Access Control (MAC) layer.